The present invention relates to a speaker device adapted for reproducing acoustic signals of a frequency band over 20 kHz.
There have been known heretofore such speaker devices as those shown in FIGS. 7, 8 and 9 for reproducing acoustic signals of a frequency band over 20 kHz.
The example of FIG. 7 shows a dynamic speaker device wherein its magnetic circuit includes a doughnut-shaped magnet 1, first and second magnetic yokes 2, 3 composed of a magnetic material such as iron, and a magnetic gap 4. The first magnetic yoke 2 includes a cylindrical center pole 2a and a discoidal flange 2b orthogonal to the center pole 2a. 
The second magnetic yoke 3 is termed a plate which is shaped like a doughnut whose inside diameter is greater than the outside diameter of the center pole 2a by a length corresponding to the magnetic gap 4.
In a state where the center pole 2a is inserted into the inner hollow portion of the magnet 1 and the inner hollow portion of the plate 3, the magnet 1 is attached fixedly while being held between the upper surface of the flange 2b and the lower surface of the plate 3. The contact portions of the magnet 1 are bonded to the upper surface of the flange 2b and the lower surface of the plate 3.
In order to reproduce signals of a frequency band over 20 kHz, a speaker diaphragm 7 of the speaker device is composed of an adequate acoustic diaphragm material having a great modulus of elasticity so as to raise the split vibration start frequency as high as possible. For this purpose, the speaker diaphragm is composed of a selected acoustic vibration material including ceramics such as SiC or carbon graphite, or metallic one such as aluminum or titanium.
The speaker diaphragm 7 in this example is composed of the acoustic diaphragm material mentioned above, wherein a dome 7a positioned at the center and shaped to be substantially arcuate in its cross section, and an edge 7b positioned outside the dome 7a, are formed integrally with each other through a link 7c. 
Further an upper end of a cylindrical voice coil bobbin 5, which is composed of a non-conductor, is bonded fixedly with a bonding agent 9 to an inner periphery of the dome 7a of the speaker diaphragm 7, and a voice coil 6 wound around the voice coil bobbin 5 at a predetermined position thereof is inserted into the magnetic gap 4 formed between the plate 3 and the center pole 2a. Further the outer periphery of the edge 7b of the speaker diaphragm 7 is bonded fixedly to a speaker frame 8.
In the speaker device shown in FIG. 7, a current is caused to flow in the voice coil 6 as an acoustic signal is supplied to the voice coil 6, and the speaker diaphragm 7 is vibrated by the interaction of the voice coil 6 and the magnetic flux in the gap 4, thereby emitting sound from the diaphragm 7.
FIGS. 8 and 9 show electromagnetic induction type speaker devices respectively. In explaining the examples of FIGS. 8 and 9, any component parts corresponding to those in FIG. 7 are denoted by the same reference numerals, and a detailed description thereof will be omitted below.
In FIG. 8, an upper end of a cylindrical voice coil bobbin 10, which is composed of a non-conductor, is bonded fixedly to an inner periphery of a dome 7a of a speaker diaphragm 7, and a conductive one-turn ring 11 adhered to a predetermined position on the inner peripheral face of the bobbin 10 is inserted into a magnetic gap 4 formed between a plate 3 and a center pole 2a. Further a driving coil 12 is wound around a position corresponding, in the magnetic gap 4, to the outer periphery of the center pole 2a, and acoustic signals are supplied to the driving coil 12. Other component parts are structurally the same as those in FIG. 7.
When acoustic signals are supplied to the driving coil 12 in the speaker device shown in FIG. 8, the conductive one-turn ring 11 is vibrated by the action of electromagnetic induction, so that the speaker diaphragm 7 is vibrated to emit sound therefrom.
FIG. 9 shows another example wherein an upper end of a cylindrical conductive one-turn ring 13 is bonded fixedly to an inner periphery of a dome 7a of a speaker diaphragm 7, and this conductive one-turn ring 13 is inserted into a magnetic gap 4 formed between a plate 3 and a center pole 2a. In FIG. 9, any other component parts are structurally the same as those in FIG. 8. The device of FIG. 9 performs the same operation as that of FIG. 8.
In the conventional speaker diaphragm 7 composed of such ceramic or metallic material, the acoustic loss coefficient (1/Q) is extremely small as less than 0.01. For this reason, there exists a disadvantage that, in the frequency band where split vibrations are generated, the sound pressure characteristic indicates a sharp and great peak dip derived from the influence of the split vibrations.
The speaker diaphragm 7 having such dome 7a and edge 7b is produced by integrally molding a thin sheet or the like. Therefore, the link 7c between the dome 7a and the edge 7b is rendered thinner as the sheet or the like is stretched in two directions.
Further when an acoustic signal is supplied to the voice coil 6 and the driving coil 12 in the above structure where the respective upper ends of the voice coil bobbin 5, the bobbin 10 and the conductive one-turn ring 13 are bonded fixedly to the inner periphery of the dome 7a of the speaker diaphragm 7, the dome 7a and the edge 7b are vibrated with 180xc2x0 phase difference at a certain frequency while the link 7c having a small mechanical strength acts as a node, so that the sound pressure generated from the dome 7a and the sound pressure from the edge 7b at the relevant frequency cancel each other out to consequently cause a sound pressure dip, thereby deteriorating the tone quality.
The present invention has been accomplished in view of the problems mentioned above. It is an object of the invention to minimize the peak dip of sound pressure derived from split vibrations of the speaker diaphragm, and also to realize satisfactory reproduction of acoustic signals in a frequency band over 20 kHz.
According to one aspect of the present invention, there is provided a speaker device which includes a speaker diaphragm composed of a selected acoustic diaphragm material whose acoustic loss coefficient (tan xcex4) is more than 0.02 in a frequency band over 20 kHz, and including a dome positioned at the center and shaped to be substantially arcuate in its cross section, and an edge positioned outside the dome and formed integrally therewith through a link; and a conductive one-turn ring inserted into a magnetic gap and bonded fixedly, at one end thereof, to the link between the dome and the edge of the speaker diaphragm. This speaker device is capable of reproducing acoustic signals of a frequency band over 20 kHz by utilizing the split vibrations of the speaker diaphragm.
In the present invention where the speaker diaphragm is composed of a selected acoustic diaphragm material having an acoustic loss coefficient (1/Q) of more than 0.02 in a frequency band over 20 kHz, it becomes possible to minimize the peak dip of sound pressure derived from the split vibrations of the speaker diaphragm in a frequency band over 20 kHz. Further since one end of the conductive one-turn ring is bonded fixedly to the link between the dome and the edge of the speaker diaphragm, the mechanical strength of the link can be increased to consequently eliminate undesired vibrations with 180xc2x0 phase difference in the dome and the edge, hence ensuring high-quality reproduction of signals in a frequency band over 20 kHz
According to another aspect of the present invention, there is provided a speaker device which includes a speaker diaphragm composed of a selected acoustic diaphragm material whose acoustic loss coefficient (tan xcex4) is more than 0.02 in a frequency band over 20 kHz, and including a dome positioned at the center and shaped to be substantially arcuate in its cross section, and an edge positioned outside the dome and formed integrally therewith through a link; and a bobbin having a wound voice coil or an adhered conductive one-turn ring disposed in a magnetic gap, and bonded fixedly, at one end thereof, to the link between the dome and the edge of the speaker diaphragm. This speaker device is capable of reproducing acoustic signals of a frequency band over 20 kHz by utilizing the split vibrations of the speaker diaphragm.
In the present invention where the speaker diaphragm is composed of a selected acoustic diaphragm material having an acoustic loss coefficient (1/Q) of more than 0.02 in a frequency band over 20 kHz, it becomes possible to minimize the peak dip of sound pressure derived from the split vibrations of the speaker diaphragm in a frequency band over 20 kHz. Further, since one end of the voice coil bobbin having the wound voice coil or one end of the bobbin having the adhered conductive one-turn ring is bonded fixedly to the link between the dome and the edge of the speaker diaphragm, the mechanical strength of the link can be increased to consequently eliminate undesired vibrations with 180xc2x0 phase difference in the dome and the edge, hence ensuring high-quality reproduction of acoustic signals in a frequency band over 20 kHz.
The above and other features and advantages of the present invention will become apparent from the following description which will be given with reference to the illustrative accompanying drawings.